The present invention relates to the cold forming or pilgering of tubular members and particularly to a die used in such processing.
In the fabrication of tubular cladding for use in fuel elements of nuclear reactors, such cladding is normally made by pilgering of zirconium alloy cylinders. A hot extruded tubular shape is reduced in cross-sectional dimensions, both in diameter and wall thickness, by multiple pilger reductions, generally at room temperature. The pilgering apparatus uses opposed dies which have a tapered groove on the die periphery in the working area. By reciprocating the opposed dies over the tube being worked, which tube is suported on an internal mandrel, cold reduction of the tube is effected. The process is incremental in that the tube to be reduced to a smaller wall thickness and diameter is fed axially a small amount over the mandrel followed by a rolling cycle of dies forward and back. At the end of each cycle, a length of smaller tubing is produced equal to the amount fed times the elongation produced by the reduction in cross-sectional dimensions.
A significant cost factor in this operation is the life of the pilger dies. Previous work has indicated that lower stress dies, such as those formed from AISI Type H13 steel alloy, typically fail by surface spalling similar to that in bearings due to the high compressive stresses normal to the groove surface. Higher hardness dies, such as those formed from Bofors SR 1855 tool grade steel with a hardness of about 58 Rockwell C, resist spalling failures. These higher stress dies fail, however, by cracking due to the cyclic tensile stresses produced in the surface of the die groove. These tensile stresses are produced by the resolution of the working stresses against the semicircular groove surface in the transverse direction with respect to the groove axis.
A number of approaches have been identified to deal with cyclic tensile stresses in pilger dies. One is to insure that a high hardness die, such as a Bofors SR 1855 tool steel die, is case hardened rather than through hardened. Case hardening has been shown to result in a compressive rather than tensile residual stress on the surface which serves to resist the operating tensile stresses in the die groove resulting in significantly higher die life. One method for assuring the presence of a hardened case is described in published Japanese Patent Application No. 55-114872 of Westinghouse Electric Corporation, the assignee of the present invention. As described therein, a "directional quenching" heat treating process for tool steels for pilger machines involved heating a pilger die to austenitizing temperature range, selectively removing heat from the die at predetermined faster rate in the direction of the desired case than the rate of removal of heat from the balance of the die, and thereafter tempering the die. The process as therein described produced Bofers SR 1855 or AISI 52100 tool steel dies which had a case hardness of between 53 to 63 Rockwell C hardness (Rc 53 to Rc 63), of about one-half to one inch thickness, with the balance of the die having a hardness of between 35 to 45 Rockwell C (Rc 35 to Rc 45).
Another approach to deal with tensile operating stresses in pilger dies includes allowing for elastic deflection under load to produce compressive stresses in the groove area. This approach is described in my co-pending U.S. application, Ser. No. 692,811 filed Jan. 18, 1985, and entitled "Pilgering Apparatus", assigned to the assignee of the present invention, which co-pending application issued as U.S. Pat. No. 4,674,312 on June 23, 1987 and is incorporated by reference herein. As described therein, a recess is provided on the inner periphery of a rotatable pilger die, having a groove on the outer periphery, such that the die is flexed by load applied and the tensile strength in the region of the groove is controlled by compressive stress produced by the flexing due to the presence of the recess.
Case hardening of pilger dies results in a compressive residual stress in the outer peripheral region of the die. Case hardening means that only a surface layer is hardened by the martensitic reaction in steel while the bulk of the die remains in the soft, untransformed condition. Residual stresses are a consequence of thermal contraction on cooling and the volume expansion produced by the martensitic hardening reaction. That is, during the quenching operation the rapid cooling on the surface results in the martensitic hardening reaction and thus volume expansion. As the interior of the die cools it can also transform to martensite (resulting therefrom in through hardening) and continue to cool and thermally contract. If the interior of the die also transforms to martensite, the accompanying volume expansion forces the already cooled and hardened surface to be displaced outwardly resulting in residual tensile stresses in the surface of the die. If the interior does not transform but continues to thermally contract, the acompanying contraction causes residual compressive stresses in the surface of the core and case. These two conditions have been shown to have a dramatic effect on pilger die life with very inferior die lives demonstrated in through hardened dies. Residual stress and thus die life and productivity is determined primarily by the change in volume in the interior or core during quenching. Whether core volume increases or decreases is currently determined by the heat treatment procedure.